1. Field of the Invention
This invention relates to a toner used in image forming processes such as electrophotography, electrostatic recording, magnetic recording and toner jet recording.
2. Related Background Art
In electrophotography, conventionally using photoconductive materials, an electrostatic latent image is formed on a photosensitive member by various means, subsequently developed by the use of a toner to form a toner image, and the toner image is transferred to a transfer material such as paper as needed, followed by fixing by the action of heat, pressure, heat-and-pressure or solvent vapor to form a fixed image.
As a method for fixing the toner image to a sheet such as paper, which is the above final step, the most commonly available method at present is a pressure-and-heating system making use of a heat roller. According to this method, the surface of the heat roller and the toner image on the fixing-receiving sheet (sheet to which toner images are to be fixed) come into contact with each other under application of pressure, and hence the thermal efficiency in fusing the toner image onto the fixing-medium sheet is so good that the toner image can rapidly be fixed.
However, since the heat roller surface and the toner image come into contact with each other under application of pressure, there is a problem of high-temperature offset such that part of the toner image may adhere, and be transferred, to the fixing roller surface, and transferred again to the next fixing-receiving sheet to cause an offset phenomenon to contaminate the fixing-receiving sheet.
There are also increasing sever demands on fixed images in order to adapt the higher speed of apparatus, more energy saving and shorter wait time, and further to deal with the folding of fixed images, the release of a pressure-sensitive adhesive tape from fixed images, and so forth. The low-temperature fixing performance is desired to be further improved, but when improving the low-temperature fixing performance, problems are raised such that the mechanical strength of toner is apt to lower, and development durability tends to lower and cause problems such as adhesion (melt adhesion) of toner to developer carrying members and melt adhesion of toner to photosensitive members.
Accordingly, toner is required to have superior low-temperature fixing performance and high-temperature anti-offset properties, and also to have superior development durability and cause no toner adhesion (melt adhesion) to the above members (developer carrying members and photosensitive members).
Heretofore, as resins for toners, vinyl copolymers (such as styrene resins) and polyester resins have been primarily used.
The polyester resins, though having high glass transition temperature, can readily provide resins having low-softening point and, when heated and melted, show so good ability to wet fixing-receiving sheets such as paper that good fixing can be performed at lower temperature. However, on the other hand, they have the disadvantage of causing the offset phenomenon at a high temperature. Hence, it is necessary to use release agents having a variety of release properties at the time of high-temperature fixing.
For example, Japanese Patent Application Laid-open No. S57-208559 discloses a toner containing a polyester resin as a binder resin. There, however, is room for improvement in fluidity and agglomeration resistance which are powder characteristics required for toners. Also, the polyester resin is difficult to pulverize in a process involving the pulverization step and is disadvantageous in respect of productivity.
On the other hand, resins having superior releasability at high temperature include vinyl resins. The vinyl resins have such properties that the temperature at which the melt viscosity begins to lower is relatively high and high releasability can be easily obtained, but have a relatively high melt starting temperature. However, the molecular weight of the binder resin in its molecular weight distribution is reduced in an attempt to lower the temperature at which the melt viscosity begins to lower, with the result that necessary release effect is not obtained. Even if a release agent is used with vinyl resins having reduced molecular weight in order to achieve low-temperature fixing, the melted resins themselves have so low viscosity as to make it difficult to obtain the necessary release effect.
A toner is also proposed which makes use of a resin obtained by polymerizing a vinyl monomer in the presence of a reactive polyester resin and increasing the molecular weight of the polymer through cross-linking reaction, addition reaction and grafting reaction in the course of polymerization. Further, in Japanese Patent No. 2962809, a resin composition for toner is proposed which is composed of a copolymer of a polyester resin and a vinyl monomer and a polyester resin.
Toners containing the vinyl polymer or gel matter obtained by such cross-linking reaction may be improved in anti-offset performance. However, where the vinyl polymer obtained by such cross-linking reaction is used as a toner raw material, the polymer undergoes larger shear force depending on the height of viscoelasticity at the time of melt kneading in producing toner particles, and so, the cutting of polymer molecular chains is accelerated to lower the melt viscosity of the binder resin, so that the anti-offset performance of the toner at the time of fixing deteriorates. Also, the cutting of polymer molecular chains may generate heat to cause the temperature rise of the polymer itself at the time of melt kneading, thereby raising such a problem that it is unable to achieve the sufficient dispersion state of the components contained in the toner particles.
It is also known to control molecular weight distribution. For example, in Japanese Patent Application Laid-open No. H10-87837 and Japanese Patent No. 3118341, toners are proposed in which molecular weight distribution controlled to have peaks separated into two regions of low molecular weight and high molecular weight is formed and which have as a binder resin a resin composition constituted of a carboxyl-group-containing vinyl resin, using as a cross-linking agent a glycidyl-group-containing vinyl resin.
These toners exhibit superior effects on the improvement in anti-offset properties. However, there remains room for further improvement in low-temperature fixing performance and high-temperature anti-offset properties when used in a system of high process speed or when applied to simplify a fixing assembly or to achieve energy saving. In addition, when using such a cross-linked resin, the resin has high viscosity to cause coarse particles in producing toner particles. As a result, the toner tends to bring about faulty images due to sleeve coat non-uniformity, which is remarkable especially in a high-speed development system.
In Japanese Patent Application Laid-open No. H09-6050, a toner is proposed in which the ratio of weight-average molecular weight measured by a light scattering method to weight-average molecular weight measured by GPC is specified. However, when this toner is used especially in a high-speed development system, there remains room for further improvement in high-temperature anti-offset properties.
Thus, under existing circumstances, there is no toner which can satisfactorily achieve all the space saving, high speed and energy saving.